1. Field of the Invention
The present invention relates to a particulate filter regenerating system which comprises a fuel injection device adapted to inject fuel into a combustion chamber of an engine, and an exhaust gas temperature-adjusting device adapted, when a PM deposit amount which is an amount of particulate matter (PM) deposited in a particulate filter provided in an exhaust passage of the engine, reaches a predetermined value, to cause the fuel injection device to perform a post-injection for injecting fuel into the combustion chamber during an expansion stroke of the engine, so as to execute a filter regeneration process of raising a temperature of exhaust gas flowing into the particulate filter to burn the deposited PM.
2. Description of the Background Art
In a diesel engine and a lean-burn gasoline engine, particulate (particulate matter: PM) consisting primarily of carbonaceous components is contained in exhaust gas at higher levels as compared with a normal gasoline engine configured to perform combustion of an air-fuel mixture around a stoichiometric air/fuel ratio. For this reason, in the diesel engine or the lean-burn gasoline engine, there has heretofore been employed a technique of arranging a particulate filter in an exhaust passage of the engine to trap PM, and, when a predetermined amount of PM is accumulated or deposited in the particulate filter, executing a filter regeneration process for burning and removing the deposited PM.
Generally, the filter regeneration process is achieved by performing a so-called post-injection for injecting fuel into a combustion chamber during an expansion stroke of the engine. More specifically, through the post-injection, fuel components are added into exhaust gas, and burned in an oxidation catalyst or the like arranged upstream of the particulate filter, so that exhaust gas raised to high temperature flows into the particulate filter, and thereby the deposited PM is burned and removed based on functions of the high-temperature exhaust gas and a catalyst supported on the particulate filter.
As an exhaust gas purification system for an engine employing the above filter regeneration technique, there has been known one type as disclosed, for example, in JP 2004-162633A. Specifically, an exhaust gas purification system disclosed in this patent document is configured as follows. When it is determined that a difference between respective pressures detected by upstream and downstream pressure sensors provided across a particulate filter is equal to or greater than a predetermined value (i.e., the particulate filter is clogged), a post-injection (after-injection) is performed in an earlier-stage regeneration mode. Then, when a difference between respective exhaust gas temperatures before and after the particulate filter becomes equal to or less than a reference temperature difference after a predetermined time has elapsed from initiation of the post-injection in the earlier-stage regeneration mode, a post-injection mode is shifted from the earlier-stage regeneration mode to a later-stage regeneration mode. Upon the shifting to the later-stage regeneration mode, a fuel injection amount for the post-injection (post-injection amount) is reduced, or a fuel injection timing during the post-injection (post-injection timing) is advanced, as compared with that in the earlier-stage regeneration mode, to suppress a rise in exhaust gas temperature.
In the technique of the above patent document, when a difference between respective exhaust gas temperatures before and after the particulate filter becomes smaller to cause the shifting to the later-stage regeneration mode, a rise in exhaust gas temperature is suppressed by reducing the post-injection amount or advancing the post-injection timing. This provides an advantage of being able to prevent thermal damage of the particulate filter, and thermal degradation of oxidation catalyst supported on the particulate filter.
However, in the later-stage regeneration mode where the post-injection amount is reduced (or the post-injection timing is advanced), although an excessive rise in exhaust gas temperature can be suppressed, a PM burning rate becomes lower along with a reduction in exhaust gas temperature, and thereby, if the later-stage regeneration mode is continued until PM deposited in the filter is completely burned and removed, a process time in the later-stage regeneration mode is increased to cause an increase in total fuel consumption due to the post-injection, which is likely to result in deterioration in fuel economy performance.